Pre-alignment system of exposure apparatus having wafer cooling means and exposure method using the same

ABSTRACT

A wafer exposure apparatus includes a special wafer cooling unit, namely, an air showerhead, for controlling the temperature of a wafer which is to be transferred from a wafer pre-alignment system to a wafer stage of photolithography exposure equipment. The wafer which has been heated in the course of being transferred from a spin coater to the wafer pre-alignment system, and may be further heated by sensors of the wafer pre-alignment system, is cooled to the same temperature as that of a wafer stage. Accordingly, a thermal equilibrium may be rapidly established between the wafer and the wafer stage when the wafer is transferred to the wafer stage. Accordingly, excessive thermal expansion of the wafer caused by a difference in temperature between the wafer and the wafer stage is prevented. Therefore, an excessive error in aligning the wafer with the optics of the photolithography exposure equipment can be prevented.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor wafer exposureapparatus and to a method of using the same to expose a photosensitivefilm formed on the wafer.

[0003] 2. Description of the Related Art

[0004] During the fabricating of semiconductor devices, wafers aresubjected to a sequence of different manufacturing process. In thecourse of such a sequence, the wafers go through changes in temperaturefrom room temperature to a temperature of more than several hundreddegrees centigrade. Accordingly, the wafers experience thermal expansionand contraction during the processes.

[0005] Ideally, the physical characteristics of wafers should bemaintained throughout the sequence of manufacturing processes. However,the physical characteristics of wafers change due to thermal expansionand contraction thereof during the different manufacturing processes.This causes an error in aligning the wafers with optical equipmentduring photolithography. Such an alignment error, if in excess of apredetermined limit, results in a defective final product.

[0006] More specifically, in the fabricating of semiconductor devices, aphotolithographic process includes coating a wafer with photoresist, andaligning, illuminating and developing the wafer. FIG. 1 shows aconventional aligning exposure apparatus

[0007] Referring to FIG. 1, reference numeral 10 designates a spincoater for coating a wafer with a photoresist. The wafer coated with thephotoresist by the spin coater 10 is transferred to an exposureapparatus 12. The exposure apparatus 12 comprises a wafer transfersystem 14, a wafer pre-alignment system 16, a wafer stage 18 and aD-chuck 20. The wafer coated with the photoresist film by the spinner 10is transferred via the wafer transfer system 14 to the waferpre-alignment system 16. The wafer is pre-aligned by the waferpre-alignment system 16 before being transferred to the wafer stage 18.Specifically, a flat zone of the wafer is oriented in a predetermineddirection. The pre-aligned wafer is transferred to the wafer stage 18and is then accurately aligned with optics of the photolithographyexposure equipment. Thereafter, an exposure process is executed. In theexposure process, the same pattern as that carved in a reticle istransferred to the film of photoresist on the wafer. Then the the waferis transferred via the D-chuck 20 and the wafer transfer system 14 tothe spin coater.

[0008]FIG. 2 is a sectional view of part of the wafer pre-alignmentsystem 16 of the conventional wafer aligning exposure apparatus 12.Referring to FIG. 2, the wafer pre-alignment system 16 comprises aP-chuck 22 on which a wafer is loaded, and mark and edge sensors 23 and24 disposed around the P-chuck 22. The edge sensor 24 senses a flat zoneof the wafer W loaded on the P-chuck 22. The mark sensor 23 is used toorient the sensed flat zone in the predetermined direction. The edgesensor 24 generates heat while sensing the flat zone of the wafer W, andtherefore, the temperature of a portion of the wafer W exposed to theedge sensor 24 increases.

[0009] As described above, elements of the conventional aligningexposure apparatus have heat generation elements which increase thetemperature of a wafer. In the wafer aligning exposure apparatus 12, thewafer transfer system 14, the wafer pre-alignment system 16 and thewafer stage 18 have different temperature environments. For example, thewafer stage 18 consistently maintains its temperature using a specialtemperature control device. On the other hand, the wafer transfer system14 and the wafer pre-alignment system 16 use the air supplied from acleanroom to control their temperatures. Not only does air heated byvarious heat generation sources in the cleanroom flow into the systems,but also each system itself of the wafer aligning exposure apparatus 12has a heat generation element such as the edge sensor 24. Thus, theinside of each ssytem of the aligning exposure apparatus is heated.Furthermore, since different aligning exposure apparatuses are used fordifferent processes during the fabrication of semiconductor devices,different wafer transfer systems and different wafer pre-alignmentsystems are used.

[0010] For these reasons, the wafer which passes through the wafertransfer system 14 or the wafer pre-alignment system 16 in the exposureapparatus 12 is heated, and therefore, the temperature of the wafer ishigher than a temperature maintained at the wafer stage 18 when thewafer arrives at the wafer stage 18. This means that the wafer hasthermally expanded to a much greater extent than it would have thermallyexpanded at the wafer stage 18. Accordingly, the alignment error of thewafer becomes so great that the pattern of the reticle cannot beaccurately transferred to the wafer. To prevent this problem, before thealignment and exposure processes are carried out, the wafer is caused toremain at the wafer stage 18 for a long time sufficient for the waferand the wafer stage 18 to attain a state of thermal equilibrium.However, requiring the wafer to remain idle for a long period of timecompromises the productivity of the semiconductor fabricating process.

SUMMARY OF THE INVENTION

[0011] It is therefore an object of the present invention to preventexcessive thermal expansion of a wafer caused by a difference intemperature between a wafer stage and the wafer, without requiring thewafer to remain idle at the wafer stage for a prolonged period of time,thereby reducing the possibility of an alignment error withoutcompromising the productivity of the manufacturing process.

[0012] To achieve this object, the present invention forcibly cools thewafer before it is transferred to the wafer stage.

[0013] To this end, the exposure apparatus of the present inventionincludes a wafer transfer system, a wafer pre-alignment systemcomprising a P-chuck, mark and edge sensors disposed around the P-chuck,and a wafer cooling unit for cooling a wafer while it is beingpre-aligned, a wafer alignment system including a wafer stage, and aD-chuck.

[0014] The wafer cooling unit is installed over the P-chuck between themark sensor and the edge sensor.

[0015] The wafer cooling unit is in the form of an air showerhead whichsprays a cooling gas onto the entire surface of a wafer loaded on theP-chuck.

[0016] In a method of the present invention, a wafer is transferred froma spin coater to a wafer transfer system and from there to the waferpre-alignment system.

[0017] The cooling of the wafer is preferably performed at thepre-alignment system while the wafer is being pre-aligned and until thetemperature of the wafer becomes equal to a predetermined temperatureprevailing inside the wafer alignment system.

[0018] The wafer is then transferred to the wafer stage of thealignement system where it is aligned with the optics of thephotolithography exposure equipment and is then irradiated with lightproduced by a light source of the exposure equipment, whereby aphotosensitive film formed on the wafer in the spin-coater is exposed.

[0019] As described above, the present invention forcibly cools thewafer in the wafer pre-alignment system to make the temperature of thewafer equal to that of the wafer stage just before it is transferred tothe wafer stage. Consequently, the thermal expansion of the wafercoincides with that which would occur at the wafer stage, therebypreventing wafer alignment errors from occurring.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The above and other objects, features and advantages of thepresent invention will become more apparent by referring to thefollowing detailed description of the preferred embodiment thereof madewith reference to the attached drawings, of which:

[0021]FIG. 1 is a block diagram of a conventional wafer aligningexposure apparatus;

[0022]FIG. 2 is a sectional view of part of a wafer pre-alignment systemof the conventional wafer aligning exposure apparatus;

[0023]FIG. 3 is a block diagram of an embodiment of a wafer aligningexposure apparatus according to the present invention;

[0024]FIG. 4 is a sectional view of part of a wafer pre-alignment systemof the wafer aligning exposure apparatus according to the presentinvention;

[0025]FIG. 5 is a sectional view of a wafer cooling unit of the waferpre-alignment system according to the present invention;

[0026]FIGS. 6 and 7 are a plan view and a bottom view of the wafercooling unit; and

[0027]FIG. 8 is a flowchart of an embodiment of an exposure methodaccording to the present invention, using the exposure apparatus of FIG.3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] The present invention will now be described more fully withreference to the accompanying drawings. The same reference numeralsdesignate like elements throughout the drawings. Furthermore, thepresent invention is applicable to any basic photolithographic processused for fabricating semiconductor devices. As a matter of convenienceand again, for the sake of clarity, the present invention will bedescribed with reference to a stepper-type of exposure apparatus.

[0029] Referring now to FIG. 3, an exposure apparatus 40 of the presentinvention includes a wafer transfer system 42, a wafer pre-alignmentsystem 44, a wafer stage 46 and a D-chuck 48. The wafer pre-alignmentsystem 44 includes a wafer cooling unit 44 a.

[0030] As a wafer from the spin coater 10 passes through the wafertransfer system 42 and the wafer pre-alignment system 44 before arrivingat the wafer stage 46, part of the wafer is heated by an edge sensor ofthe wafer pre-alignment system 44. The wafer cooling unit 44 a serves tocool the wafer before the wafer is transferred to the wafer stage 46. Inother words, the wafer cooling unit 44 a cools the wafer, which is beingheated until it arrives at the wafer pre-alignment system 44, to apredetermined temperature, preferably, to the temperature of the waferstage 46. To this end, the wafer cooling unit 44 a comprises an airshowerhead.

[0031] Referring to FIG. 4, the wafer pre-alignment system 44 includes aP-chuck 52, onto which a wafer 54 is loaded, disposed at a lower portionthereof. A mark sensor 55 extends around part of the P-chuck 52, and anedge sensor 56 is disposed opposite the mark sensor 55. Once the wafer54 is loaded on the P-chuck 52, the edge sensor 56 senses a particularportion thereof, namely, a flat zone of the wafer 54. The mark sensor 55is then used to orient the flat zone of the wafer 54 so as to face in apredetermined direction.

[0032] As shown in FIG. 4, the wafer 54 is loaded on the P-chuck 52 suchthat parts of the peripheral edge of the wafer 54 are surrounded by themark sensor 55 and the edge sensor 56. In other words, the mark sensor55 and the edge sensor 56 both have areas facing the top and bottomsurfaces near the periphery of the loaded wafer 54. A wafer cooling unitfor cooling the wafer 54 is provided over the P-chuck 52 such that thewafer 54 is interposed between the wafer cooling unit and the P-chuck52. As mentioned above, the wafer cooling unit 58 is an air showerhead58. A cooling gas, for example, air, is ejected towards the surface ofthe wafer 54 by the wafer cooling unit.

[0033] Referring now to FIG. 5, the air showerhead 58 includes an airduct 72, namely, a cooling gas inlet pipe through which cooling gasflows, a gas ejection portion 76 from which the cooling gas is sprayedover the entire surface of the underlying wafer 54, and a necked portion74 (hereinafter “neck”) extending between the air duct 72 and the gasejection portion 76. The neck 74 of the air showerhead 58 has a reducedcross-sectional area which causes the cooling gas flowing into the gasejection portion 76 from the air duct 72 to rapidly flow over the entiresurface of the wafer 54. A plurality of holes 78 are formed in a bottomplate of the gas ejection portion 76 which faces the wafer 54. The holes78 usually have the same diameter, but the diameters of the holes mayvary as circumstances dictate.

[0034] Referring to FIG. 6, the shape of the gas ejection portion 76 ofthe wafer cooling unit when viewed from the top is circular, with theexception of first and second recesses H1 and H2 defined at the outerperiphery of the wafer cooling unit. The first and second recesses H1and H2 respectively accommodate the mark sensor 55 and the edge sensor56. As a result, the wafer cooling unit faces the entire surface of thewafer 54 except at the areas covered by the sensors 55 and 56. As shownin FIG. 7, the holes 78 are arrayed uniformly over the entire surface ofthe bottom plate of the gas ejection portion 76 of the wafer coolingunit which faces the wafer.

[0035] Next, a method of exposing a semiconductor wafer using theexposure apparatus of the present invention will be described withreference to FIG. 8. In a first step, Step 100, a wafer is coated in thespin coater 10 with a photosensitive material, for example, aphotoresist, to form a photosensitive film thereon, and then thephotosensitive film is baked at a predetermined temperature until thephotosensitive film is in a state suitable for light exposure.

[0036] After the photosensitive film coating and baking step iscompleted in the spinner 10, the wafer is transferred to the waferpre-alignment system 44 by the wafer transfer system 42. There (referback to FIG. 4) the edge sensor 56 senses a flat zone of the wafer 54.The mark sensor 55 is then used to orient the flat zone of the wafer 54so as to face in a predetermined direction.

[0037] More specifically, the edge sensor 56 comprises optical sensorelements 56 a facing both sides of the wafer 54. The optical sensorelements 56 a shine light toward the bottom of the wafer, at theperiphery thereof, in the process of sensing for the flat zone of thewafer 54. During this process, a portion of the wafer 54 passing throughthe edge sensor 56 is exposed to the light, whereby that portion of thewafer 54 attains a higher temperature than other portions of the wafer54.

[0038] Next, the wafer 54 is cooled using the wafer cooling unit 44 a,namely, the air showerhead 58 provided over the P-chuck 52 of the waferpre-alignment system 50. At this time, the wafer is preferably cooled toa temperature of the wafer stage 46 disposed downstream of the waferpre-alignment system 44. (Step 102).

[0039] If the wafer were not cooled before being transferred to thewafer stage 46, the wafer would have to remain idle at the wafer stageor the possibility of an alignment error occurring would increase.

[0040] The third step, Step 104, is a wafer aligning and illuminatingstep. More specifically, the wafer which has been pre-aligned and cooledby the wafer pre-alignment system 44 is transferred to the wafer stage46 where it is aligned with the optics 120 of photolithography exposureequipment 100 and exposed to light focused thereon by the optics 120. Inthis respect, the mechanism of the alignment system for aligning thewafer on the wafer stage with the optics 120 of the photolithographyequipment is well-known per se and as such, will not be described herein any detail. Now, a thermal equilibrium exists between the wafer 54and the wafer stage 46 because the wafer has already been cooled to thetemperature of the wafer stage 46 by the wafer pre-alignment system 44.Therefore, the wafer aligning and illuminating step may be carried outrapidly. Moreover, an excessive alignment error caused by a differencein temperature between the wafer 54 and the wafer stage 46 is prevented.

[0041] The fourth step, Step 106, comprises transferring the wafer 54,which has been completely exposed, to a spinner via the wafer transfersystem 42. Specifically, the irradiated wafer is transferred to theD-chuck 48. Thereafter, the wafer 54 is transferred from the D-chuck 48to the spin coater 10 via the wafer transfer system 42.

[0042] Although the present invention has been described with referenceto the preferred embodiment thereof, various changes thereto andmodifications thereof will be apparent to those skilled in the art. Forexample, the bottom plate of the gas ejection portion 76 having theholes 78 formed therein may be replaced by a permeable membrane whichallows the cooling gas to pass therethrough. Therefore, the true spiritand scope of the present invention encompasses all such changes andmodifications as defined by the appended claims.

What is claimed is:
 1. A wafer exposure apparatus comprising: a wafer pre-alignment system including at least one wafer sensor disposed beside said P-chuck at a location which will face a surface of a wafer supported on the P-chuck, and wafer cooling means for cooling a wafer supported on the P-chuck and heated by heat generated by said at least one sensor; a wafer stage disposed downstream of said wafer pre-alignment system in the apparatus; and photolithography exposure equipment for exposing a wafer supported on said wafer stage to light.
 2. The apparatus of claim 1, wherein said at least one wafer sensor comprises a wafer edge sensor for sensing for the presence of a peripheral edge of the wafer, and a wafer mark sensor, and said wafer cooling means is disposed over said P-chuck between the mark sensor and the edge sensor in the wafer pre-alignment system.
 3. The apparatus of claim 1, wherein the wafer cooling means is a showerhead which sprays a cooling gas over the entire surface of a wafer supported on the P-chuck.
 4. The apparatus of claim 3, wherein the showerhead comprises an air duct, a gas ejection portion through which the cooling gas exits the showerhead, and a necked portion extending between the air duct and the gas ejection portion, said necked portion having a horizontal cross-sectional which is less than that of said gas ejection portion.
 5. The apparatus of claim 4, wherein said gas ejection portion has a bottom plate facing said P-chuck, said bottom plate having a plurality of holes therethrough.
 6. The apparatus of claim 5, wherein the plurality of holes are arrayed uniformly over the entire surface of said bottom plate.
 7. The apparatus of claim 6, wherein all of the holes in said bottom plate have the same diameter.
 8. A wafer exposure method comprising: coating a wafer with a photosensitive material so as to form a photosensitive film thereon; baking the photosensitive film; subsequently transferring the wafer to a wafer alignment system; once the wafer arrives at the alignment system, aligning the wafer with optics of photolithography exposure equipment, and then illuminating the wafer at the alignment system with light from a light source of the photolithography exposure equipment; and forcibly cooling the wafer before transferring the wafer to the alignment system, whereby errors in the aligning of the wafer with the photolithography exposure equipment due to thermal expansion of the wafer are reduced.
 9. The method of claim 8, wherein the cooling of the wafer is performed until the temperature of the wafer becomes equal to the temperature prevailing inside the wafer alignment system.
 10. The method of claim 8, and further comprising placing the wafer in a predetermined orientation before the wafer is transferred to the wafer alignment system to thereby pre-align the wafer, and wherein the cooling of the wafer is performed simultaneously with the pre-aligning of the wafer.
 11. The method of claim 8, wherein the cooling of the wafer comprises spraying a cooling medium onto the surface of the wafer.
 12. The method of claim 10, wherein the cooling of the wafer comprises spraying a cooling medium onto the surface of the wafer.
 13. The method of claim 10, wherein the positioning of the wafer comprises optically examining the periphery of the wafer.
 14. A wafer exposure method comprising: transferring a wafer from a spin coater to a wafer transfer system; using the wafer transfer system to transfer the wafer to a wafer pre-alignment system; positioning the wafer in a predetermined orientation to pre-align the wafer, and forcibly cooling the wafer in the wafer pre-alignment system; transferring the wafer, which has been pre-aligned and forcibly cooled in the wafer pre-alignment system, to a wafer stage; aligning the wafer on the wafer stage with optics of photolithography exposure equipment; and illuminating the aligned wafer on the wafer stage with light produced by a light source of the photolithography exposure equipment.
 15. The method of claim 14, wherein the cooling of the wafer is performed until the temperature of the wafer becomes equal to the temperature of the wafer stage.
 16. The method of claim 14, wherein the cooling of the wafer comprises spraying a cooling medium onto a surface of the wafer.
 17. The method of claim 14, wherein the positioning of the wafer comprises optically examining the periphery of the wafer. 